The invention relates to a particle detector for detecting secondary charged particles (electrons or ions) in analytical instruments or scanning electron microscopes, and, in particular, is concerned with a particle detector that can be used effectively when the specimen under examination is relatively large.
In a scanning electron microscope, for example, which is one of the principal uses of the invention, the specimen would traditionally have a transverse dimension of 1 to 2 cm., and would present no special problem in locating a detector sufficiently close to all parts of such a specimen as to effectively detect the secondary electrons emitted from the various different parts of the specimen as the primary beam is scanned over a bombardment area of the specimen. Recently, however, it has become convenient to use a scanning electron microscope to examine substantially larger specimens, e.g. large silicon wafers and the like, which may have transverse dimensions of the order of up to 10 cm. The distance between the final lens of the electron microscope and the specimen needs to be kept short for good resolution, and the space available for the detector will therefore be limited. As a result, the main elements of the detector will necessarily be located beyond the lateral limits of the lens and the specimen. Yet the detector will be required to detect secondary electrons that are generated in the specimen at a bombardment area determined by the primary electron beam which emerges from the centre of the lens. In other words a detector in such a situation is required to have a much longer "reach" into the confined space between the specimen and the lens than has previously been necessary. Ideally this reach should be acquired without significant loss of performance.
Examples of analytical instruments in which a similar problem is posed by the increasing size of specimens and overcrowding of probes, detectors and specimens, and to which the present invention applies, are electron, ion or particle probe instruments with any combination of mass spectrometers, X-ray detectors, light detectors, back scattered electron detectors, secondary electron detectors, energy spectroscopes for chemical analysis or other electron energy analysers, Auger analysers and other analysers of the type that use an ion gun. Each of these instruments, as it scans an incident beam or ray over the specimen, causes different bombardment areas of the specimen to emit secondary electrons, back scattered electrons or ions, positive or negative. The detector collects and counts these secondary charged particles. The detector output and the scan coordinates are supplied to a computer which uses this data to produce an image or "signature" of the specimen from which much desired information can be obtained.